The present invention is directed to a system for controlling the seating velocity of an engine valve and, more particularly, to a system for controlling the seating velocity of an engine valve in a variable valve actuation system.
Internal combustion engines typically include a series of valves that control a flow of gases into and out of the engine. An engine may include a series of intake valves that control the flow of gases into a series of combustion chambers. The engine may also include a series of exhaust valves that control the flow of gases from the combustion chambers.
An internal combustion engine may also include a cam assembly that controls the actuation timing of the engine valves. A cam assembly typically includes a series of cams that are rotated in coordination with a crankshaft of the engine to cause the intake and exhaust valves to open at certain points in the operating cycle of the engine. For example, the intake valves associated with a combustion chamber may be opened when the piston disposed in the chamber moves through an intake stroke and the exhaust valves associated with the combustion chamber may be opened when the piston moves through an exhaust stroke.
A valve return spring acts on each engine valve to maintain a connection between the engine valve and the rotating cams. Each valve return spring typically exerts a significant force on the engine valve to ensure that the connection with the particular cam is not lost. The force of each valve return spring also acts to close each engine valve when the shape of the particular cam allows.
One approach to improving the overall efficiency of an internal combustion engine involves adjusting the actuation timing of the engine valves when the engine is experiencing a certain set of operating conditions. For example, the actuation timing of the intake and/or exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a xe2x80x9clate intakexe2x80x9d type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston. Selective implementation of a timing variation, such as the late-intake Miller cycle, may lead to an improvement in the overall efficiency of the engine.
However, to vary the actuation timing of an engine valve, the engine valve must be selectively disconnected from the associated cam. This may be accomplished, for example, by a hydraulic actuator or another device that forms a controllable link between the cam and the engine valve. For example, as described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, an engine valve actuation system may include a hydraulic actuator that establishes a hydraulic link between the cam and an intake valve. When the link is established, the valve will be actuated according to the shape of the cam. However, when the hydraulic link is broken, such as by opening a control valve to release fluid trapped between the intake valve and the cam assembly, the force of a valve return spring causes the engine valve to close. Thus, breaking the hydraulic link allows the engine valve to close at an earlier timing than would be achieved by the shape of the cam.
When the link between the cam and the engine valve is broken, the return spring acts on the valve to close the valve. As the cam assembly is not opposing the force of the valve return spring, the engine valve may close with a substantial force. The impact generated upon the closing of the engine valve may result in damage to the sealing elements of the engine valve. Any of this type of damage to the engine valve may prevent the valve element from properly sealing the combustion chamber. This may negatively impact the efficiency of the engine and result in a need for increased maintenance.
The disclosed system solves one or more of the problems set forth above.
In one aspect, the present invention is directed to a velocity control system that includes a housing defining an opening, a chamber adapted to receive a fluid, a first fluid passageway connecting the opening with the chamber, a second fluid passageway connecting the opening with the chamber, and a third fluid passageway connecting the opening with the chamber. A piston is slidably disposed in the chamber and is moveable from a first position to a second position. The movement of the piston from the first position to the second position forces fluid from the chamber at a first flow rate during movement of the piston through a first travel distance and at a second flow rate during movement of the piston through a second travel distance. The first flow rate is greater than the second flow rate. A check valve is adapted to prevent fluid from flowing from the chamber through the first fluid passageway when the piston is moving from the first position to the second position.
In another aspect, the present invention is directed to a method of controlling the velocity of an engine valve. A cam assembly is operated to move an engine valve between a first position where the engine valve prevents a flow of fluid and a second position where the engine valve allows a flow of fluid. A piston is moved from a first position to a second position to operatively engage an end of the piston with the engine valve to prevent the engine valve from returning to the first position. The piston is slidably disposed in a chamber of a housing. Fluid is provided to the chamber through a first fluid passageway, a second fluid passageway, and a third fluid passageway in the housing during movement of the piston from the first position to the second position. Fluid is released from the chamber at a first flow rate during movement of the piston through a first travel distance from the second position to the first position. Fluid is released from the chamber at a second flow rate during movement of the piston through a second travel distance from the second position to the first position. The second flow rate is less than the first flow rate.